Dividing square columns into triangular prisms
Easiest way is to go back a little and erase part of what you have done f you really want them as separate prisms.

Erase all but the bottom faces, search for the operator Poke Faces Alt+P and it will automatically break them into triangles.



If you need each triangle extruded as a separate prism you need an additional step before extrusion.

Select all diagonals, using the Select Similar Shift+G operator > Direction option and then rip the edges with the Rip operator V. Do the same for the opposite direction and the square subdivisions if you had not done so before yet.



Then just extrude them back up to create the prisms